Double curved shells, usually of metal, are widely used to store fluids such as water, liquefied gases and liquid petroleum products. The double curved shells are generally spherical or spheroidal in shape. At times it is necessary to place a coating on the shell interior surface. The coating can be a paint, polymeric film or an insulating layer such as one made of a polymeric material foamed-in-place, especially polyurethane foam.
Because of the large size of many of the shells it is difficult to properly coat their interior surface because of a lack of support for workmen and essential coating equipment. Present methods employ substantial scaffolding, walk ways and ladders, none of which is easily supported or assembled in the shell because of its shape.
Garis et al U.S. Pat. No. 3,548,453 and Larsen U.S. Pat. No. 3,991,842 disclose apparatus for insulating the exterior vertical walls of cylindrical tanks but they do not disclose a method or apparatus for coating the surface of a double curved shell, either on the interior or exterior surface.
Bellafiore et al pending U.S. patent application Ser. No. 176,185 filed Aug. 7, 1980, now U.S. Pat. No. 4,333,973, discloses a vehicle-like machine for applying foamed-in-place insulation on substantially flat or sloped surfaces using a reciprocating member which applies the foam in adjacent parallel strips or bands in overlapping or side-by-side arrangement.
The Dow Chemical Company apparently produces spheres and hemispheres of rigid plastic foam by a method identified as spiral generation. The method uses a specially designed machine which bends, places and bonds pieces of plastic foam together into a predetermined shape. The machine head is mounted on a boom which swings around a pivot, laying and bonding layer upon layer of foam board in a rising spherical form. The Dow machine use is believed limited, however, to producing walls for truly spherical structures or spherical sections and is apparently not used to coat the shell. Also, since the boom length is apparently constant it would be unsuitable both for forming nonspherical sections which do not have circular sections through the vertical axis and for coating the interior surface. This is because the working head of the boom could not be maintained a uniform distance from a nonspherical shell wall with a constant length boom.
From the above discussion it is believed clear that a need exists for novel apparatus and methods which can be used to coat double curved interior surfaces of shells, such as spherical and spheroidal shells.